This invention relates to digital communication systems, and more particularly, to a communication system, and related method, that uses a voice compression algorithm for digitally compressing an audio signal.
Telephone providers are accustomed to perform audio channel testing through analog cellular channels using test software at a Mobile Telephone Office (MTSO). The tests cause a specially configured mobile subscriber unit, placed at each cell site, to loop back received audio signals, send test tones, perform channel quality measurements on the forward audio path, and transmit test results.
More recently, mobile cellular providers have provided digital cellular channels that transmit audio signals using digital coding techniques. A class of speech coding techniques provides high-quality synthesized speech at relatively low bit rates. An example of such a coding technique is the Vector-Sum Excited Linear Prediction (VSELP) compression algorithm, used in the North American Digital Cellular Standard, IS-54B. Voice compression algorithms such as these do not process single frequency tones accurately. This is due to short periods of filter instability when a tone is processed. Generally, a short period of filter instability does not greatly affect the quality of voice signals because voice signals have a constantly varying pitch. The filter instability effects are further tolerated because this class of voice compression algorithms allow transmission of quality voice signals at greatly reduced data rates.